1. Field of the Invention
This invention relates to an image signal processing device for processing image signals and more particularly to an image signal processing device of the kind having an image memory arranged to temporarily store an input image signal and, after that, to output the image signal stored.
2. Description of the Related Art
Some of the conventional image signal processing devices are provided with image memories which are arranged to temporarily store input image signals and, after that, to output the image signals stored.
The image signal processing devices of the above-stated kind include, for example, an electronic still video camera. An FIFO (first-in/first-out) memory is popularly employed as the image memory for the electronic still video camera, because it requires a relatively few number of wiring connections; it is controllable by a simple memory controller; it permits data writing and data reading at a high speed; etc.
The conventional image signal processing device using the FIFO memory as a frame memory is arranged as follows: in storing an image signal for one frame, a first-field image signal is stored in a first-half address area among addresses of the frame memory and a second-field image signal is stored in a latter-half address area.
However, in repeatedly reading out an image signal for one frame stored in the frame memory by every field, the conventional image signal processing device does not allow a second-field image signal to be repeatedly read out, although a first-field image signal can be repeatedly read out, because the frame memory is an FIFO memory. If the frame memory is used, for example, as a field memory, the frame memory allows the writing or reading of only an image signal for one field despite of its storage capacity of storing an image signal for two fields. In a case where field image signals of two different kinds are to be temporarily stored and the stored field image signals of two different kinds are outputted by switching them from one kind over to the other, therefore, the field image signal of one kind must be stored and read out separately from the field image signal of the other kind. It is thus impossible to make instantaneous switch-over of one field image signal over to the other field image signal. The operability of the conventional image signal processing device, therefore, has been very poor.